The present invention generally relates to a shutter control circuit and, more particularly, to a strobe control circuit for producing a strobe trigger signal to provide a proper exposure in flash photography.
It has been customary heretofore that, for generation of a strobe trigger signal relative to a shutter in flash photography, required data including a distance to a subject, a film speed and a guide number of a strobe flash unit is input in the form of an analog voltage or current to a shutter control circuit, and a calculation thereof is executed in an analog mode to determine the time to produce a trigger signal. In such a system where an analog signal is processed, the performance is prone to vary depending on the temperature characteristic, voltage characteristic and so forth of each element, hence requiring a circuit to compensate for such variation.
In some films developed of late, a multiplicity of contacts are provided on the surface of a cartridge, and the film speed is encoded in accordance with a particular pattern of such contacts and is automatically read out by a group of switches. For computing such film speed in an analog mode by the conventional method described above, it is necessary to execute digital-to-analog conversion prior to the computation, so that problems naturally arise with respect to the conversion accuracy and contact resistance of the switches. According to this system, the time obtainable by analog-mode computation with respect to input shooting data is limited merely to one changing arithmetically (as 1/2, 1/3 . . . in accordance with increase of the film speed multiplied by 2, 4 . . . ) or to one changing geometrically (as 1/2, 1/4 . . . ). Consequently, the opening formed by sectors needs to be proportional thereto. Thus, it becomes impossible to apply the above system to any shutter where the sector opening is not in a fixed relationship with respect to time.